A "ground plane" is commonly defined as a conductive layer in a circuit board that is electrically contiguous in its X and Y dimensions relative to signal conductors in adjacent layers of the circuit boards. The signal conductors can be viewed as lines drawn over and under the plane of the ground plane. Among other function (e.g., distribution of DC power), the ground plane provides a low impedance path for return currents of signals and for shielding.
A "backplane" as commonly defined is an assembly that provides interconnection functions for a plurality of separate electronic modules (e.g., circuit boards or daughter cards) that connect to it. In some products such as the Series 900, a backplane may be expandable by the addition of modules or sections. This requires the interconnection of conductors in the separate modules to pass signals between modules as if the separate modules were part of one large backplane. The interconnection can be accomplished by using commonly available connectors.
Most electronic busses require that there be a ground plane in the backplane. When separate modules are connected together, it is important to also interconnect the ground plane between the modules. While a signal conductor on a backplane has a significant impedance of about 75 ohms, the ground plane requires an impedance of 0.1 ohms or lower to operate properly. Signal carrying conductors are often specified in ohms of impedance at a signal frequency of interest, and is implied to be uni-directional along the conductors length. Ground planes are specified in ohms of impedance per square at a frequency of interest, and is omni-directional in the plane. Although ground planes are omni-directional conductors, the impedance of an interconnection between ground planes can be considered as uni-directional (in the direction of the interconnection).
If ground planes on separate modules are connected with impedances above 0.1 ohms, signal return currents between the ground planes can develop voltages across the ground plane interconnection that may result in the system failure. The characteristic impedance of the contact system used in connectors and the characteristic impedance of the signal conductors in the backplane have to be matched so that signals pass through the connector with minimal signal degradation. Impedance in contacts systems usually range from 30 ohms to 150 ohms, where 75 ohms is approximately the average impedance.
Some conventional ground plane interconnection systems use continuous conductor strips to connect ground planes together. The continuous conductor strips are usually made of some type of metal, such as copper, for example. The impedance of the continuous conductor strips is within the operational impedance range from near 0 to 0.1 ohms. However, continuous conductor strips add significantly to the cost of the connector and to the size and complexity of the mechanical connections. Accordingly, there is a significant need to connect two ground backplanes together without requiring any external mechanical device or continuous conductor strips other than using a regular connector and that operates within the acceptable impedance range.